1. Field of the Invention
The present invention relates to a crystal oscillator and particularly to a system and method of using both open-loop, temperature-based and closed-loop frequency estimate based techniques for pulling the frequency of that crystal oscillator.
2. Description of the Related Art
A wireless personal handy phone system (PHS) frequently uses crystal oscillators to generate a frequency for controlling reception and transmission of signals between terminals. Unfortunately, a crystal oscillator generates a frequency that can vary due to changes in ambient temperature. To overcome this problem, many PHS transceivers include a crystal oscillator as well as associated components to provide temperature compensation of the output frequency. This type of crystal oscillator circuit is called a temperature compensated crystal oscillator (TCXO).
These TCXOs are typically quite accurate, e.g. within as little as +/−0.1 ppm (parts per million) versus +/−20 ppm for a non-TCXO. (Note that a frequency tolerance is the amount of frequency deviation from a particular center frequency at ambient temperature, e.g. referenced at 25 C. This frequency tolerance is defined using a maximum and minimum frequency deviation specified in ppm.) However, the accuracy of such TCXOs is offset by their corresponding manufacturing cost. For example, a TCXO could add upwards of $1 per station, thereby significantly adding to the cost of manufacturing a wireless device. Moreover, recently implemented communication systems can use less exacting frequency calibration. For example, PHS (which is increasingly being used in Asia) can provide high-quality voice and data transmission at low cost, but requires carrier frequency accuracy only within ±3 ppm and digital system clock accuracy of ±5 ppm. Therefore, a need arises for a system and method of tuning the frequency of a crystal oscillator with an accuracy usable in a PHS system.